Acoustic diaphragm



July 23, 1929. c. A. HARTMANN 1,721,625

ACOUSTIC DIAPHRAGM Filed May 2'7, 1927 Imm- Karl FRET- T H ifTrE'aTfi Patented July 23, 1929.

UNITED STATES PATENT OFFICE.

CARL ALBERT .HARTMANN OF BERLIN -SIEMENSSTADT, GERMANY, A'SSIGNOR TO SIEMENS & HALsxE GERMANY.

Application filed may 27, 1927, Serial No.

This invention relates to an improved type of acoustic diaphragm for use in telephone receivers and transmitters and the object of the invention is to produce a diaphragm which is very light in weight and at the same time most efficient. g It has been proposed previously to use very thin metal foil as a diaphragm, the reason being that it was necessary to make the vibrating systems as lightas possible. lVhen made singly in' the laboratory such diaphragms can be made,'but as a commercial article of manufacture'they are not fesistant enough and require complicated auxiliary devices which make manufacture very difficult. Y Y

This invention overcomes this difficulty, and by proceeding with the conviction that lightness of the vibrating system is an essential feature, the present diaphragm is made of material having a very low ,specific gravity, like paper, silk tissue, etc. It is therefore possible to make such diaphragms substantially thicker and still keep them to the same weight per unit area of about one milligram per cubic centimeter. In this instance papers of 0.005 thickness have. suflicient strength so that a diaphragm made from it can be satisfactorily constructed as a factory article without any special devices being employed.

A diaphragm constructed of the above material, will of course, have no magnetic or electrical conductivity, which is one of the first requisites when used in telephone receivers or transmitters. Accordin to the invention, this diaphragm is there ore subjected to a treatment which imparts electric or magnetic conductivity to it. purpose use is made of the otherwise unde.

sirable property .of absorbing moisture to make the material electrically conducting. This property maybe assisted or produced by impregnating the diaphragm with hygroscopic salts or solutions, calcium chloride (CaCl for instance. v

' A'paper diaphragm so treated can be used with success in the unstretched condition in condenser transmitters or telephone receivers. Hitherto in order to. place the natural frequency of the diaphragm the other side the range of audibility, it was necessary to either stretch the diaphragm very tightly or 7 allow the diaphragm to cushion a certain For this Acotrsrrc DiAPI-IRAGM.

194,856, and in Germany October '22, 1926.1

volume, of air; at times both means were used at the same time. first method it is necessary to use such intense stretching forces that as a rule the elastic limit of the diaphragm isxexceeded .even though it consists of steel, bef rea natural vibration of 10000 Hertz is reached. When using the second expedient,such small'volumes of air are necessary that the ratio of When using the surface to the volume of the air cushion exceeds the highest admissible value; this is characterized by the fact'that the volume changes are no longer adiabatic but that during a compression, heat is noticeably led off. The air cushion then acts no longer in a manner which increases the natural frequency, but with an increasingly damping action.

Owing to these necessarily small air volumes other difficulties also occur, especially during the transmission of low frequencies.

As is known, all notes of low frequency,

say under 100 Hertz, with subjectively equal sound intensities have up to twice the amplitude of the sounds of higher frequencies. Such amplitudes are, however, considerably impaired by an air cushion of the. usual thickness of a few, tenths or even hundredths of a millimeter; under certain circumstances the diaphragm may even hit against the back of the air cushion.

. With an unstretched diaphragm according to the invention, the natural vibration of the slight weight may be easily shifted beyond the range of audible frequencies. When using an aircushion the danger of distortion of the low, frequencies and of the striking of the diaphragm is eliminated with certainty by the fact that substantially thicker air cushions, say 5 millimeters thick can be used. Further the system is much more sensitive' than those formerly knownas the air cushion, being thicker, the tone is softer and the diaphragm can make greater amplitudes owing to itslig htne'ss and greater. freedom 'of motion.

The electrical conductivity of the diaphragm'mass can also be obtained by applying an exceedingly thin layer of finely divided conducting material like graphite.

.The use of the-new diaphragm in electromagnetic apparatus such as telephone receivers and loud speakers can be effected in known manner by applying a magnetic armature to the diaphragm. Owing to the very small mass of diaphragm which is to be set in motion, the armature itself may-be made very light and small, so thatits mass only causes a slight increase of the average mass of the vibrating system. For agiven diaphragm diameter of 40 millimeters such as is usual for normal telephones, a diaphragm with fairly thick paper of 0.013 millimeters and specific gravity of approximately 1 will weigh about 18 milligrammcs. For this diaphragm an iron armature of 5 millimeters diameter and 0.05 millimeters thick is .perfectly adequate, this weighing only about 8 milligrammes. The armature weight is thus less than half of the diaphragm weight so that the advantageous effect of the light diaphragm is scarcely impaired.

According to the invent-ion it is further possible to dispense with the armature alto gether by making the diaphragm magnetically conductive by impregnating it with a para-magnetic salt solution say of ferric chloride (Fe Cl When using very thin papers of. about 0.006 millimeters any small holes which are apt to be present (leleteriously affect the apparatus they spoil the efficiency of the air cushion. This can be remedied easily by impregnatingthe paper with a weak solution (about of collodion.

Fig. 1 shows a front View of the diaphragm, partly in section.

Fig. 2 shows a cross sectional View of the diaphragm.

The accompanying single sheet of drawings illustrates the invention. It shows, on an enlarged scale, a cross section and a front view of the disclosure. Reference numeral 1 shows a diaphragm of paper or other suitable material as specified in the foregoing a deposit of collodion; and reference nu- V meral 5; a magnetic armature attached to the diaphragm.

lVhat is claimed is 1. An acoustic diaphragm for telephones or the like, comprising a, sheet of fibrous material impregnated with a solution of calcium chloride to impart electric properties thereto.

2. An acoustic diaphragm for telephones orthe like, comprising a. sheet of fibrous material with highly hygroscopic properties, said hygroscopic properties utilized for impregnati-ng said diaphragm with a solution of calcium chloride to impart electric properties thereto. v

3. An acoustic diaphragm for telephones or the like consisting of a disk of fibrous material evincing hygroscopic properties and impregnated with a solution of calcium chloride to impart electrical conducting properties tosaid diaphragm.

4. An acoustic diaphragm for telephones or the like, comprising a sheet of paper impregnated with a solution to impart electrical propcrties'thereto and coated with a weak solution of collodion to make said diaphragm air-tight.

5. An acoustic diaphragm for telephones or the like, comprising a sheet of fibrous material impregnated with a solution of calcium chloride to impart electrical properties thereto and coated with a Weak solution of collodion to make said diaphragm air-tight.

' CARL ALBERT HARTMANN. 

